2nd or 3rd?

Were the Tonnants and Bucentaurs considered 2nd or 3rd rates?

3rd rates

As far as I am aware they are third rates, second rates are 90 guns + but less than 100, plus they are two deckers so third rate.

Had a quick look in my French Warships in the Age of sail, the Tonnant/Bucentaur are in the two decker section, so again classed as third rates

I totally concur gentlemen. Definitely third raters.
Rob.

This guide also agrees:

https://boardgamegeek.com/thread/190...ps-sails-glory

So, is a second rate three decks with 90-99 guns, then? Is this really a meaningful distinction?

What do you mean by meaningful?

It was a military distinction, same as a Battleship, pocket Battleship, Heavy Cruiser in WW2 parlance.
I am sure DM can give a more detailed distinction for you but from my reading the rating system was used for size and power, the bigger the ship the greater the number of long guns ( carronades were not usually counted for the number of guns ), the more expensive hence the higher the rate.

Remember 3rd rates were initially 64 gun ships, these were improved until the 74 became the norm , but some nations, upgunned some 2 deckers to 80s.

first rates were over 100 guns, the 2nd rates were the fillers between the two, not powerful enough to be a first rate, but too powerful to be a common third rate. Same with expence, they cost more to equip and provision.

I could not have explained it better myself Chris.
Rob.